By LOUISE STORY

Published: September 27, 2006

IT was only a matter of time before a homemade commercial appeared during the Super Bowl.

And now it is happening.

Frito-Lay and Chevrolet, looking to capitalize on the buzz created by user-generated content, are holding ad-making contests, with winners getting money and prizes and their spots used as part of Super Bowl XLI campaigns. Some of the spots will also appear online, giving them a chance to be spread around the Internet virally.

In addition, the National Football League expects to announce rules for its own ad contest in the next month.

The Super Bowl spots, unlike some other recent online user-involvement campaigns, will be heavily vetted by the companies and their ad agencies. Frito-Lay will allow consumers to vote for which advertisement should be shown, from five finalists the company will choose. Chevrolet and its ad firm will choose which ad idea to use themselves.

''In both cases, it's a very tight grip,'' said Drew Neisser, president and chief executive of the Renegade Marketing Group, a marketing firm based in New York that is affiliated with Dentsu. Frito-Lay and Chevrolet ''are reserving judgment because they have a brand to protect.''

Commercials not chosen for the Super Bowl by the two companies may still be posted online on YouTube or individual blogs, Mr. Neisser added. ''The risk is that the consumer will tell you what they really think,'' he said.

User-based content carries plenty of risks, especially at such a high-profile event. Thirty seconds of commercial time during last season's Super Bowl cost advertisers $2.5 million, and some of the most prominent advertisers buy several spots.

Still, the game, on CBS on Feb. 4, presents marketers with one of the largest television audiences available, and advertisers are especially interested in such events these days because they are usually watched live rather than recorded. More than half of households with televisions watch the game each year, according to the N.F.L., and last year an average of 90.7 million people were watching the game at any given point.

Frito-Lay is accepting video submissions through Dec. 1. Five of the submitted commercials will then be posted on Yahoo Video, and consumers will vote to help determine the one shown during the game. The five finalists will receive $10,000 prizes, and the winner, of course, will also receive valuable air time.

''We know that millions of videos are created and posted online everyday, so with that said, we're expecting quite a number of ads to pick from,'' said Jared Dougherty, a spokesman for Frito-Lay, which is a division of PepsiCo.

User-generated content, now common on popular sites like MySpace and YouTube, includes videos, photos and songs created by nonprofessionals and shared with the world online. Advertisers have increasingly incorporated amateur content into their marketing plans over the last year because of the form's popularity.

Chevrolet's ad contest was meant to reach 18- to 25-year-old consumers, said Stuart Pierce, director of advertising and sales promotions for Chevrolet. The company's Super Bowl commercial will feature four car models marketed to younger buyers.

Chevrolet, a division of General Motors, made waves in March when it released video clips online of its 2007 Tahoe sport utility vehicle and encouraged Internet users to add their own text to the images. However, some of the resulting commercials, which were sent around the Internet in e-mail and blogs were critical of the Tahoe.

This time around Chevrolet will retain more control over the ad for the Super Bowl. The car company is soliciting ideas only for its spot during the game. The actual commercial will be produced by professionals. Chevrolet has received about 600 registrations for the contest and will continue accepting registrations from college students through the end of this week.

The company will select the idea for its spot with its ad firm, Campbell-Ewald, an Interpublic Group firm based in Michigan. Then Campbell-Ewald will include the students who created the winning idea in the entire production process, Mr. Pierce said.

Chevrolet will also show an additional commercial during the game that does not involve amateurs in the creation process, he said.

These user-influenced Super Bowl ads represent a move of user-generated content into mainstream television. While there are television channels like Current TV that broadcast user-generated videos, most user-generated content so far has been shown only on the Internet until recently.

The Country Music Television network recently announced a campaign for viewers to create commercials that the station will broadcast, and Nikon USA recently chose 16 consumers to shoot photos with the new Nikon D80 camera. About 20 of the photographs will be used in a print campaign for Nikon.

Super Bowl advertisers who stick with professional content for their spots next February may later release their commercials online to amateurs.

Personiva, a media firm in San Francisco, is encouraging prominent Super Bowl advertisers to post their ads on a collective Web site designed by Personiva after the game. The site will allow viewers to put images of themselves into the Super Bowl spots. Personiva will charge Super Bowl advertisers a flat fee to participate in the site plus a fee of 10 cents each time someone uses the ad, said Yogesh Sharma, Personiva's chief executive.

Personiva created similar Web sites for Hewlett-Packard and Levi Strauss this past summer. On average, ads created on those sites are being e-mailed to three people after they are created, Mr. Sharma said.

''Consumers really have strong brand preferences and when they prefer a brand, they begin to associate themselves with that brand,'' he said. ''We enable this kind of brand evangelism.''

Photo: Five amateur Super Bowl ads in a contest will be posted on Yahoo. A vote will help determine which one should be shown during the game.